Miles of airlines: when is it convenient to buy them?

A person can start earning miles with an airline once they join their loyalty program. The idea is that each time they travel, they provide the frequent flyer number when booking the flight or when they check-in, and thus the miles will be automatically credited to their account.There are loyalty plans that, in addition to accumulating, allow them to buy miles. However, there are certain restrictions that travelers can find when trying to acquir…